Lift trucks, also known as forklifts, are commonly used to raise and position heavy loads on elevated surfaces. Some lift trucks include a reach carriage that extends horizontally away from the mast of the lift truck to “reach” a position that is within the reach carriage's limit of extension. Such lift trucks are referred to herein as “reach trucks.” Typically, a reach truck's reach carriage includes a pantographic extension arm connected between the mast and the fork assembly. Extension and retraction of the extension arm is conducted using a hydraulic ram cylinder attached between the mast and a pivot point of the extension arm. The extension arm extends as the cylinder fills with fluid, and retracts as the cylinder empties.
Considering that, during operation of the extension arm, the fork assembly may support a load of several tons significantly far off the ground, close control of the extension arm velocity may be needed to avoid abrupt stops at the extension arm's limits. In particular, it would be advantageous to gradually or incrementally slow the extension arm as it approaches its fully extended or fully retracted positions. This may be done manually by the operator if the reach truck has operator controls therefore. However, this method may be imprecise, and the extension arm position could be ignored or miscalculated by the operator.
Previous approaches for controlling the reach carriage velocity include continuous positional feedback encoders and specially-machined hydraulic ram cylinders, both of which have significant drawbacks. Encoders track the position of the extension arm with a continuously-operating servo motor, and provide position data to the reach truck's electrical systems to control flow rate into and out of the cylinder. Encoders add significant manufacturing cost, contributed by both the cost of the encoders themselves and the cost of designing mechanical and electrical operations to include the encoders. Encoders are also subject to wear and damage due to their continuous operation, and add replacement costs. Specially-machined hydraulic ram cylinders endeavor to control the flow rate of fluid into the cylinder with permanent contained structures. These devices also carry a high cost, and further may be significantly more complex than a typical hydraulic ram cylinder. The complexity invites manufacturing defects and inconsistencies, as well as machine surface and linkage failures due to relatively poor tolerance.
It would therefore be desirable to incorporate automated systems and methods to control the velocity of the reach carriage without adding significant cost and complexity to the reach truck.